Apparatus for reactivating radio tubes



Jan. 7, 1947. 7 c. HELLIAR 2,413,707

APPARATUS FOR REACTIVATING RADIO .TUBES Filed July 25, 1945 Patented Jan. 7, 1947 UNITED STATES PATENT" OFFICE CyriiLHelliar', Vancouver, British Columbia, Canada ApplicationlulyZE, 1945,.S'erial-1No. 606,956

(01. tie-28')- 6 Claims.

This invention relates to apparatus for reactivating radio tubes having thoriated tungsten or oxide coated filaments.

Thoriated tungstenfilaments are composed-of a mixture of tungsten with about 1 to 2% thorium and a little carbon to act as a reducing agent. In normal operation they have a. very thin substantially monomolecular thorium coating on.the surface,from. which electron emission takes place. The normal operating temperature of the filament is such that as the coating is reduced by electron emission, it is replaced by more thorium which comes to the surface. Oxide coatedfilaments are generallyformed by applying barium or strontium carbonates to a metallic core, for example platinum or Konel. metal, and are originally activated by conversion of some of the carbonate to oxide and some of they oxide to-metal, the electron emission taking place from the latter.

A thoriated tungsten or oxide coated filament may cease to operate because of some sudden large flow of electrons whichwill-cause it to lose its; metallic emissive coating atsuch a high rate that the coating is not replaced as quickly as it islost. p r The purpose of reactivation is to reconstitute an emissive metallic coating if any emissive. materialis left in the filament. The usualmethod of reactivating a thoriated' tungsten filament is first to flash the filament for 10 to.20 seconds at 3 to 3%-times normal filament voltage to. drive off from the filament any non-uniform thorium coating, and then. to apply about 1. /2 times normal. filament voltage forv about an hour for the-purpose of heating the filament sufliciently tov bring a new thorium coating to its surface. The usual method of reactivating an oxide coated filament is. first to apply about twice normal voltage for 2 to 4 minutes in order to convert some of the carbonate to'oxide, andthen to apply a voltage about 10% above normalfor lto 2 hours inrorder-to convert some of the oxideto metal.

The difficulty of these normal methods of re.- activation is that the condition of the tube to which the method is applied is unknown and can only be ascertained by comparatively elaborate tests. Accordingly,for. practical purposes it is necessary to preselect conditions on the basis of an average used tube. under. treatment has above. average activity, then the filament will be heated above normal operating temperature for too long and deactivation will set in. If, on the other hand; it, has below average activity,v then the filament will If the tube actually not be heated above normal operating tempera-.- ture for long enough. andmaximum reactivation will not be obtained. It is in practice. impossible with. present methods. to reach maximum reactivation in any tubes whose activity does not happen to approximate that of the average used tube. In a great many cases reactivation. may be'alinost negligible. and in other cases the tube may have less activity after. treatment than it had before. treatment.

According. to the, present invention the abovementioneddisadvantages are avoided and an apparatus is provided by meansof which maximum reactivation may be automatically obtained without riskof going. beyond that maximum to deactivation. The invention depends essentially on applying across the tube a D. C. voltage substantially. above normal plate voltage from. a sourcehaving very low voltage regulation, and supplying, substantially normal current in the filament heating circuit.

Apparatus according to the invention for reactivating a radio tube having a filament and a plate and a grid therefore comprises means for supplying substantially normal heating current to the filament, a generator of D. C. voltage substantially higher than the normal plate voltage of the tube, this generator having an internal resistancesubstantially greater than the normal internal resistance of the tube, and means for connecting, this generator across the tube, normally from plate to cathode; The normalinterna'l resistance of the tube means; in this application, the resistance between plate and filamen't whichthe tube was designed by the manuf'act'urer to have. Similarly the normal plate voltage is'that with which the tube was designed bythe manufacturer to operate.

With the apparatus of the invention the extra heating necessary to produce the required'emission comes, not as. a result of any increased voltage in. the filament heating circuit, which cannot be satisfactorily adjusted for the conditions prevailing. in the particular tube under treatment, but rather as a'result of the electron flow across the tube started" by the highplate voltage applied. This. plate voltage, however,

owing to the low voltage regulation in the genera'to'r,v automatically decreases as the current through thetube increases andapproaches normal. The possibility thereforev of. carryingjthe treatment beyond" maximum reactivation to deactivation may be substantially eliminated wholly automatically, since. the internal. resistanceoi' the generator may be. so. chosen that; when the internal resistance of the tube reaches normal, that is when reactiviation has been completed, the plate voltage will be substantially normal. The rise in current through the tube in reactivation is comparatively slow but the drop in current as deactivation sets in is very sharp (see Morecroft, Principles of Radio Communication 2nd ed., p. 444, Fig. 6). If the Voltage regulation is high, the time during which the current across the tube is at maximum is extremely short, being reckoned in milliseconds, with the result that deactivation would set in and proceed to a substantial extent before any indicating instrument could warn the operator to disconnect the tube.

The voltage of the D. C. generator is not critical, provided that it is substantially higher than the normal plate voltage of the tube. If it is too low, the reactivation process will take an inordinately long time. The more nearly dead is the tube to be treated, the higher the voltage should be. Satisfactory results may, in many cases, be obtained with a voltage as low as twice normal voltage. Ordinarily, it is preferable that the voltage should be from about three to about six times the normal plate voltage of the tube. The upper limit is simply that the voltage must not be so high as to break down insulation of the tube under treatment.

It is preferable that the generator should give a pulsating D. C. voltage, since such a voltage gives in effect a series of shocks and will start electron flow from the cathode where a constant voltage of the same value might not have the required efiect. Accordingly, the preferable D. C. generator is a half Wave rectifier which may be supplied through a transformer from an ordinary 110 volt main. A half wave rectifier is preferable to a full wave rectifier because the shock effect of the former is greater and its voltage regulation is lower.

The invention will be described in somewhat more detail by reference to the attached drawing which shows diagrammatically an example of an apparatus according to the invention.

In the drawing, the primary 2 of the transformer l is connected to an ordinary electric supply, for instance 110 volts. The transformer has three secondaries, 3, 4 and 5. Secondary 3 is in circuit of the cathode 6 of a half wave rectifier 1 to provide heating current for the latter. The plate 8 of the rectifier is connected to the secondary 4, which is provided with a number of contacts 9, 9a and 91), for the purpose of supplying a.

variable high voltage for the rectifier circuit.

The secondary is likewise provided with a number of contacts l0, Illa and lb. The tube II under treatment has a filament l2, a plate I3, control grid 14, screen grid l5, and a suppressor grid I6 connected to the filament inside the tube. The heating current for the filament I2 is provided by connecting the filament in circuit with the secondary 5 of the transformer through a switch I! which may be moved to one of the contacts l9, Illa and lb, the purpose of having a number of contacts being to enable the supply of the filament heating currentwhich is substantially normal for the particular tube under treatment.

nal resistance of the rectifier 1 is so chosen as to be substantially greater than the normal internal resistance of any tube for the reactivation of which the apparatus is designed. Generally, the internal resistance of the generator should be at least about two or three times as great as the normal internal resistance of the tube to be treated. I

In order to avoid a negative space charge in the tube, it may be desirable to apply a positive bias to the control grid M. This may be done by opening the switch l9 so that the plate current is then supplied through the resistance 21, and by closing the switch 28 onto the contact 29, thus connecting the control grid M to the point 30 through the wire 3|, contact 29, switch 28, and wire 32.

In the case of a tube such as that shown, which includes a screen grid, it may sometimes be desirable for the purpose of initiating discharge through the tube to connect the generator initially across the tube between screen grid and filament rather than between the plate and filament. This is provided for in the apparatus shown, where switch [8 may be opened and the switch 33 closed thus connecting the screen grid l5 to the rectifier through the wire 34, switch 33 and wire 23. With the apparatus shown it is possible to have both the plate and the screen grid at the same voltage by closing both of the switches l8 and 33.

If the activity of the tube to be treated is very low it may be desirable to connect the generator initially across the tube between the control grid and filament. In this case a considerably lower Voltage must be used than in the case where the connection is between the plate and filament. In the apparatus shown the control grid may be connected to the rectifier by closing the switch 28 onto the contact 35, the connection then being by the wire 32, switch 28, contact 35, wire 36 and The rectifier 1, which constitutes the D. C. gen:

erator, may be connected across the tube between the plate and filamentby closing the switch l8 to establish connection with the plate, and by closing the switch l9 and movingthe switch 20 to the appropriate one of the contacts 9, 9a and 9b to establish the required plate voltage. The interwire 23. If such a connection is to be made, the switch 29 will first be moved to, say, the contact 9 'to give the lowest voltage across the tube. When electron flow has started between the filament and control grid the switch 28 will be opened or closed onto the contact 29, the switch 29 will be moved to a contact giving a higher voltage across the tube, and the switch I3 will be closed to connect the plate l3 to the rectifier.

It will be appreciated that the apparatus shown in the drawing is merely illustrative of various forms of apparatus which might be used, and that theinvention is not confined to the reactivation of pentodes such as that illustrated in the drawing. It may be adapted for the reactivation of any radio tube having a filament, grid and plate. The filament may be either of the type which itself carries the emissive material or of the type which'heats an indirectly heated cathode having the emissive material.

This application is a continuation-in-part of my previous application Serial No. 484,264, filed April 23, 1943. a

What I claim is:

1. Apparatus for reactivating a radio tube having a filament and a plate and a grid, comprising means for supplyingsubstantially normal heating current to said filament, a source of D. C. voltage substantially higher than the normal plate voltage of said tube, said source having an internal resistance substantially greater than the normal internal resistance of the tube, and means connecting said source across said tube. v 2. Apparatus according to claim 1, wherein the voltage of the source of D. C. voltage is from about three to about six times the normal plate voltage of the tube.

3. Apparatus according to claim 1, wherein the internal resistance of the source of D. C. voltage is about two to three times the normal internal resistance of the tube.

4. Apparatus according to claim 1, wherein the source of D. C. voltageis a half-Wave rectifier.

5. Apparatus according to claim 1, comprising 10 means for connecting the source of D. C. voltage across the tube between the plate and filament.

6. Apparatus according to claim 1, for reactivating a radio tube having a screen grid in addition to a filament and control grid and cathode, comprising means for connecting the source of D. C. voltage across the tube between the screen grid and filament.

CYRIL HELLIAR. 

